1. Field of the Invention
The present invention relates to a lighting optical system, and more particularly, to a small-sized lighting optical system with an improved performance.
2. Background of the Related Art
Technical advances in display devices have created lighter, thinner and larger screens.
Examples of such display device include projectors or big-screen projection TVs.
In recent years, micro devices have been introduced as a new technology for projectors and projection TVs. Examples of micro device include backlit LCD (HTPS) panels, reflective LCoS (Liquid Crystal on Silicon) panels, and DMD (Digital Micromirror Device) panels.
Depending on how many micro devices are used, the projectors or the projection systems can be categorized as single-panel, double-panel, and triple-panel projectors or projection systems.
In case of the backlit LCD (HTPS), triple-panel optical systems are widely used. As for the LCoS triple-panel, double-panel and single-panel optical systems are all available. As for the DMD, single-panel optical systems are generally used. However, it should be noticed that each of these examples does not always have advantages only.
For instance, a triple-panel optical system for the backlit LCD (HTPS) has too many optical elements and includes a relay lens for compensating optical path differences.
Besides the relay lens, the backlit LCD uses an X-prism for combining colors.
Since there is an optical path difference among R, G and B lights, there was a need to develop a new optical system capable of compensating the optical path difference. That was how the X-prism got involved therein.
FIG. 1 is a schematic diagram illustrating a triple-panel projection system according to a related art.
Referring to FIG. 1, the triple-panel projection system includes a lamp 110 for emitting light; fly-eye lenses 120, 121 for splitting the light emitted from the lamp 110 into micro lens cell units; a PBS (Polarizing Beam Array) 130 for forming incident light into a linearly polarized light with one optical axis; condensing lenses 140, 142, 142 for condensing light; dichroic lenses 151, 152 for splitting the light into R, G and B colors; LCDs (Liquid Crystal Displays) 161, 162, 163 for providing or displaying R, G and B images; mirrors 171, 172, 173 for changing a traveling path of each of the split lights by the dichroic lenses 151, 152 so that the split lights travel to their corresponding LCDs 161, 162, 163, respectively; relay lenses 181, 182 disposed on the traveling path of light to focus the light to a desired position; and an X-prism 190 for combining images from the LCDs 161, 162, 163, respectively.
The following will now explain the operation of the related art projection system described above.
The light from the lamp 110 incidents on the first dichroic lens 151.
The first dichroic lens 151 then reflects red light and transmits cyan light.
The red light reflected from the first dichroic lens 151 is reflected again by the mirror 171 and incidents on the first LCD 161.
The second dichroic lens 152 reflects green light and transmits blue light.
The green light reflected from the second dichroic lens 152 incidents on the second LCD 162.
The blue light transmitted through the second dichroic lens 152 goes through the relay system 200 consisting of the first and second relay lenses 181, 182, and the first and second mirrors 172, 173, and as a result the light path difference of the blue light from the red and green lights is compensated. Afterwards the blue light incidents on the third LCD 163.
The incident lights on the first, second and third LCDs 161, 162, 163 experience phase modulation according to an input signal and put corresponding image information therein. The lights containing the image information are then emitted from the first, second and third LCDs 161, 162, 163, are synthesized through the X-prism 190, and reach a screen (not shown) by means of a projection lens (not shown).
The above-described projection system (or projector) requires a number of optical elements for splitting and combining colors.
It also requires the relay system for compensating the light path of the blue light to make it equal to those of the red and green lights.
Because of the relay system the size of a light engine is increased, more optical elements are required, and a great number of elements should be adjusted to array an optical axis.
Meanwhile, the red and blue lights were in the same polarization state to obtain a maximum light efficiency taking advantage of the X-prism, but the green light had a different polarization state. Therefore, a retardation plate was additionally installed next to the X-prism to make the green light have the same polarization state with that of the red and blue lights.